Monostable oscillator control



May 22, 1956 BY; l

ATTORNEY Unitedl States Patent MON OSTABLE OSCELLATOR CONTROL LowelliE. Norton, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 22, 1951, Serial No. 233,034

The terminal 15 years of the term of the patent to be granted has been disclaimed 5 Claims. (Cl. Z50-36) and (where m is a master oscillator signal frequency and B is an offset frequency), it is preferable, and in some instances necessary, to monostably control the signal frequency thereof.

Monostable oscillator control is particularly applicable to microwave frequency stabilization with gas spectral lines. The signal frequency of the above-mentioned output oscillator which, by way of example, may be adjusted to may be utilized as a local oscillator signal to heterodyne with a given frequency signal to provide an intermediatefrequency signal useful in said stabilization.

In present phase-lock frequency control systems it is apparent that an instantaneous deviation in the desired output signal frequency, say an increase in frequency from www where rb is a time dependent phase angle, causes a control signal to be produced for correcting the output frequency to the desired value. Since the frequency separation between and is not necessarily great and since the oscillator may be biestable, the control signal produced by certain transient frequency shifts may shift the output oscillator signal frequency to the undesired stable condition *ice Obviously, since the output oscillator is incorrectly stabilized, an incorrect intermediate-frequency signal is obtained which is unsatisfactory for spectral line frequency stabilization purposes.

The present invention affords monostable frequency control of bi-stable oscillators by providing a phase lock frequency control system` which includes phase shift means for producing frequency control signals indicative of both the magnitude of the instantaneous output signal frequency variation and of the desired one of the aforementioned two stable operating conditions.

An object of the invention is to provide a system for monostably controlling the signal frequency of a normally bi-stable oscillator.

Another object of the invention is to provide an improved phase lock system for monostably controlling the signal frequency of a normally bi-stable oscillator.

A further object of the invention is to provide an im-l proved frequency control system for use with a micro- Wave gas spectral line frequency stabilization system.

According to a typical embodiment of the invention, the signal output from a master osciilator is coincidently applied to a beat detector circuit and to a rola phase shift circuit. The phase shift circuit output signal is then applied to a second beat detector. The desired output signal, requiring monostable frequency control thereof, is then simultaneously separately heterodyned with the generated and the phase shifted master oscillator signals. Separate modulation signals derived from the heterodyning are then compared in phase with the phase of a given signal from which comparison control signals are obtained of proper sense to monostably control the frequency of the aforementioned desired output signal.

A second embodiment of the invention is provided wherein the above-mentioned phase shift circuitry is utilized periodically. Since some oscillators are more stable than others, a synchronous switch may be utilized which controls the rate at which control signals are derived for monostable control. Use of the synchronous switch enables the system to operate with a single signal comparison channel and hence substantially simplifies the control circuitry.

The invention will be described in greater detail with reference to the accompanying drawing in which Figure l is a schematic block diagram of a prior art phase lock frequency control system; Figure 2 is a schematic block diagram of a first embodiment of the invention, in which a phase shift circuit is provided for monostably controlling the operating condition of a normally bi-stable oscillator; and Figure 3 is a schematic block diagram of a second embodiment of the invention in which a phase shift circuit is periodically utilized for monostable frequency control purposes.

Like reference characters are applied to like elements throughout the drawing.

Referring to Figure 1 of the drawing, a prior art phase lock frequency control system is illustrated. A brief description of the operation of this conventional system is useful in appreciating the novel features and advantages of the instant invention.

A first (or master) oscillator 1 generates an output signal of frequency and w1 -B 2n' The signals generated by these two oscillators l and 3 are mixed together in a beat detector 5 from which the difference modulation signal of frequency is passed by a filter unit 7 to a phase comparison detector 9 wherein the modulation signal phase is compared with a standard signal of frequency is applied to said oscillator for the frequency control thereof.

Since both of these output conditions are stable, as above shown, the system is unable to distinguish between them and consequently operation may occur at the undesired one of the two conditions for the following reasons.

Assume that the output from the master oscillator is e1=E1 sin w11 (l) and that the output from the output oscillator is e3=E3 Sin (ufl-Bh (2) The cross product of e1 and es, due to the heterodyne action in the beat detector 5, is

kExEg COS (w1 iB w1 tkEgE3 COS (w1 :i: B -l-wt:

kElEa COS (B) kElEg GOS (2w1:i::.B)t

Since only the first term (the difference frequency) is retained by filtering, the useful term is eu=kE1E3 C025 It is apparent that cos Bt: cos (-Bt) is an identity. Hence the difference frequency signal for a desired output signal frequency of is Aindistinguishable from the difference frequency signal obtained at the second stable condition Lul-B 21r The need then clearly exists in several systems, including the cited example of the spectral line frequency stabilization system, for a phase lock system in which operation rnay occur at only one signal frequency` With reference to Figure 2, and according to the invention, the master oscillatorl again provides an output signal of frequency The output oscillator 3, as before stated, operates at It should be noted, however, that the master oscillator 1 may be modulated. In either case the output oscillator 3 follows in step with frequency variations of the master oscillator 1 so that the normal frequency displacement is always When the output oscillator drifts from its desired operating frequency, the instantaneous output voltage obtained therefrom is ea=Ea sin [(w1iB)t-{rb] where ,b is again an arbitrary phase angle which, in general, is a function of time. Since the instantaneous master oscillator output voltage is e1=E1 sin w11, the difference frequency cross product term derived from applying the two signals e1 and es to a beat detector 5 is k eF-Ezlii @Us (it-w) --vtrtun3 Goegnies/)1 The potential e5 and a reference potential e1i=E11 cos Bl, produced by a standard phase oscillator 11, are applied to a phase comparison detector 9 from which the D.C. component of the detector output is output signal to the beat detector 5, the master oscillator signal is also applied to a phase shift circuit 13 from which the signal is obtained. Heterodyning en with the output oscillator signal e3 in a second beat detector 5 produces, by filtering and retaining the difference frequency modulation signal, the signal The important result to be observed is that if the desired output oscillator signal frequency is wl-i-B 21T then e5'=+kE1E3 sin (BH-5b); but, if the desired signal frequency is e5'=kE1Ea sin (Bt-1lf). Hence, e5 either leads or lags es by phase which lead or lag is dependent upon the sign of the output oscillator signal wli 2n The reference potential en derived from the previously mentioned standard phase oscillator 11, is successively coupled to a second phase shift circuit 18 and to a second phase comparison detector 9 wherein its phase is compared with the phase of the second beat detector signal e. The D.C. component of the detector output is then (6) where m1 and m2 have the same significance as before and the plus or minus value depends upon whether the output oscillator signal frequency is ygreater or less than the master oscillator signal frequency.

The variational part of Equation 5 has the same dependence on for either output condition wliB 21r The variational part of Equation 6, however, has either a positive or negative cosine dependence on tlf, depend-ing upon which of the two output operating frequency conditions wliB 21r is selected.

It is this difference in the outputs of rthe two phase comparison detectors 9 and 9 which is used to distinguish between the stable output condition wl-l-B 21r and the second stable output condition wl-B 2n' It is possible to utilize the outputs of the two detectors 9 and 9 additively for frequency control but this is not preferred since it then becomes necessary to balance out the non-variational term 2-E9. This follows since the additive output is either for oscillator frequency uffi-B 2nfor frequency w1 -B 21r ln the second case there is no variational voltage and no frequency control. In the first, and operative, case the non-variational term 2E9 must be balanced out. For this reason the two detector outputs are` combined and added in opposed sense in an output adding circuit from which frequency control signal of either @15:0', or e15=2m1m2E9 cos tb for the two frequency conditions wl-l-B 21r and wl--B 211' respectively, may be obtained.

Therefore, if the additive .connection is used, one of the two possible frequency conditions wliB 21r gives a constant voltage output independent of gl/ and hence no frequency control, while the other frequency condition gives a control voltage which varies as cos 6 plus aconstant terni which serves no useful purpose, and, in general, must be balanced out. If the preferred phase opposition connection is used, one of the two possible frequency conditions wliB 21r provides zero control voltage independent of tb, while lthe other frequency condition provides a control voltage proportional to cos 5b. Hence, no matter which connection (additive or subtractive) is chosen, only one of the two frequency conditions wliB 21r produces a control voltage and provides frequecy control. Therefore, there is no ambiguity in output frequency. Either frequency wliB 21x' may be produced by choice of the sense ofeither of the 1r/2 phase shift circuits 13 or 18. Control will occur at either 50:11-/2 or depending upon the polarity of the control voltage conneet-ion. Again, `there is no ambiguity because the algebraic sign of the control voltage-phase characteristic at has opposite sense to the slope at 31r H- Z A filter 17 is provided for removing undesirable modulation product terms and noise prior to the application of the frequency control signal to the output oscillator. In Figure 3 a switched system is disclosed, according to the invention, for monostably controlling the output oscillator 3 of Figure 2. The signal of frequency obtained therefrom is applied to a beat detector 5 wherein, at a predetermined rate comparison detector 9 wherein it is compared in phase with a signal of frequency generated by a standard phase oscillator 11 for half of the switching cycle due to the synchronous switch 19, and is compared in phase during the second half of the switching cycle with` a vr/Z phase shifted signal due to a second phase shift circuit which is connected Ito the standard phase oscillator 11. The signal obtained from the .phase the square wave output from beat detector will be zero at #/:11-/2 and at Taking first the condition centering on \,b=1r/2, when the phase starts to slip between \,b=1r/ 2 and rb=0 the square wave has one sense, but if the slippage is between rr/2 and 1r the sense is reversed. Hence, the control signal subsequently developed and applied to the oscillator providing the Wanted output signal affords stable operation at only one output condition.

For the condition centering on .the sense of appearance of a square wave output voltage of one sign or the other with smaller or larger values of p than reverses.V Therefore, lfor a particular polarity connection of the frequency control voltage, control will be at either but both are not stable.

Output frequencies at either may be selected at will by reversal of the 1r/ 2 phase shift due to the phase Shifters 13 and 1.8.

The comparison detectors herein utilized ygenerally may be of any type but a prefer-red one is described in my copending application Serial No. 228,921, filed May 29, 1951 now Patent No. 2,683,218 issued July 6, 1954. The vsynchronous switch 19, above cited, may be any suitable double pole, double throw switch. At micro- Wave frequencies a switch which may be desirable is shown and described by D. K. Bishop in the Wireless Engineer, vol. XXIV, No. 282, pp. 67-70 (1947).

In summation it may be seen that the herein disclosed invention provides monostable frequency control of a normally bi-stable oscillator in that frequency control signals derived according to the invention are sensitive to the desired one of the two stable operating conditions.

The system recited with reference to the circuitry of Figure 3 is particularly desirable when the rate at which the output signal frequency is inspected is moderate since only one beat detector and one phase comparison detector are required. A further advantage of the switched sys- 8 tem is that no potential unha-lance occurs in using a single signal comparison channel. In the event that the switching rate is too high for the switched system to function satisfactorily, the circuitry disclosed with reference to Figure 2 maybe employed.

What is claimed is: 1. A system for monostably controlling the frequency of a normally bi-stable oscillator comprising, a first oscillator for providing an output at frequency a second and normally bi-stable oscillator for providing an output at frequency uri-B first and second beat detectors, means for applying portions of the outputs of said first and second oscillators to -said first beat detector, a phase shifter, means for applying another portion of the output of said first oscillator to said phase shifter, means for applying the output of said phase shifter and another portion of the output of said second oscillator to said second beat detector, a first phase comparison circuit coupled to said first beat detector to receive the output of said first beat detector, a second phase comparison circuit coupled to said second beat detector to receive the ou-tput of said second beat detector, a -third oscillator providing an output at `frequency means for applying the output of said third oscillator to said first and second phase comparison circuits, means for combining the outputs of said first and second phase comparison circuits to produce a frequency control s-ignal, and means for applying said frequency control signal to said second oscillator to monostably control its frequency.

2. A system as claimed in claim l including a second 90 phase shifter connected between said third oscillator and said second phase comparison circuit.

3. A system for monostably controlling the .frequency of a normally bi-stable oscillator comprising, a first oscillator for providing an output at frequency wiB means for applying the output of said second oscillator to said beat detector, a comparison circuit, a third oscillator providing an output at frequency means for applying the outputs of said beat detector and said third oscilla-tor to said comparison circuit to produce a frequency cont-rol signal, and means for applying said `frequency control signal to said second oscillator to monostably control its frequency.

4. A system as claimed 'in claim 3 wherein said comparison circuit comprises a phase comparison circuit.

5. A system as claimed in claim 3 including -a second 90" phase shifter, said switch means including means for applying the output of said third oscillator in one `instance directly 'to said comparison circuit and in another instance through said second phase shifter to said comparison circuit.

References Cited in the le of this patent UNITED STATES PATENTS Guanella I une 28, 1949 Guanella et a1 Sept. 12, 1950 Hugenholtz Feb. 6, 1951 Ranger Feb. 27, 1951 

